1. Field of the Invention
The present invention relates to an ink jet printing apparatus, a printing method, a print medium and a program. More particularly, this invention relates to an ink jet printing apparatus, a printing method, a print medium and a program for forming an image on a print medium by dividing an image of one color into a plurality of images of the same color and printing the divided images as the print medium is transported using a plurality of line heads arranged along a transport direction of the print medium.
2. Description of the Related Art
In repetitively printing fixed images, such as business forms, in large volumes, a dry electrophotographic printer has been used. In recent years, an ink jet printing apparatus has come to be spotlighted as a printer that can take the place of the dry electrophotographic printer. The ink jet printing apparatus in general can handle a large quantity of printing processing because it directly prints on rolled paper. Because of this capability and its low running cost, the ink jet printing apparatus is suited for printing various kinds of business forms, such as application forms for insurance, notification slips of utility fees and application forms for mail-order purchase.
A line type ink jet printing apparatus, which uses a line head having a large number of ink ejection nozzles arrayed in a direction perpendicular to the transport direction of a print medium, is known to be particularly suited for a high-speed printing of such materials as pasteboards, label or tag sheets, and business forms among others. A commonly used line type ink jet printing apparatus to form a color image has a plurality of line type ink jet print heads (also referred to simply as line heads) arranged in the transport direction of the print medium so that different line heads eject different color inks to form a color image. That is, one line head produces an image of one color. Therefore, in the commonly used line type printing apparatus, its maximum printing speed is determined by a maximum drive frequency of each line head.
In other words, the general ink jet printing apparatus cannot realize a printing speed in excess of the maximum drive frequency of a single line head (a maximum nominal frequency at which ink can be repetitively ejected in one second while keeping a stable image quality). Therefore, the general line type printing apparatus has a problem of being unable to meet the market demand for an improved printing speed.
Aside from the above-described general line type ink jet printing apparatus, another line type ink jet printing apparatus has been proposed which divides an image of one color into a plurality of images of the same color and prints them with a corresponding number of print heads. This printing apparatus rasterizes the image data of one color, divides the raster data generated, and allocates the divided raster data to a plurality of line heads for printing. Unlike the general line type ink jet printing apparatus, which forms an image of one color with one print head, this ink jet printing apparatus with multiple line heads can print an image at a frequency in excess of the maximum drive frequency of one line head. This allows for a printing operation at a higher speed than is possible with the conventional printing apparatus (for example, see Japanese Laid-Open Patent Application No. 2005-238556, which corresponds to Japanese Patent Application No. 2004-049447).
The high-speed line type ink jet printing apparatus, however, has a problem that, should any one of a plurality of line heads ejecting the same color ink deteriorate in the ink ejection performance, a proper image printing cannot be continued. The ink ejection performance of the line head can be degraded by a variety of factors, such as heat, pressure or chemical reaction with ink during the process of ink ejection. The degree of performance degradation is not uniform among a plurality of line heads and only one of the line heads often reaches the end of its service life early. In conventional apparatus, when it is decided that only one of the print heads has reached the end of its life during the printing operation, there is no alternative but to interrupt the printing operation and replace the print head in question.
When a sub ink tank is provided for each line head, inks in the sub tanks are not necessarily consumed uniformly. Thus, if, for example, the amount of remaining ink becomes too small in only one sub tank and the print head associated with that sub tank fails to print, an operator is left with only two options: one is to continue the printing operation in a state where the print quality cannot be guaranteed, and the other is to stop the printing operation and replenish the ink.
However, it is desired that the printing operation be continued as much as possible because the line type ink jet printing apparatus normally is required to achieve a high throughput. In the event of an ink ejection failure, the continued printing operation will deteriorate the print quality. That is, continuing the printing operation and maintenance of the print quality may contradict each other.
It may be conceived to prolong the service life of the line heads and increase the capacity of the sub tanks as countermeasures against the above problem. However, there are many difficulties that need to be overcome before such countermeasures can be realized because prolonging the service life of the line heads requires a lot of time and cost for research and development and increasing the sub tank capacity will result in an increase in the overall size of the printing apparatus and a cost increase.
In printing business forms or fixed form paper, each page of the print medium such as a paper sheet is printed with a large number of lines repetitively as shown in FIG. 10. In this case, if the raster data that was generated by rasterizing the image data is divided and allocated to a plurality of line heads for printing, a problem may occur in which only a particular line head is used concentratedly and the replacement frequency of that line head increases.